What are quantum dots?

Wouldn't it be great if we could control
individual atoms? Just imagine if we could "turn" them on and off
to store bits of information, make them light up with different colors,
or control them in all kinds of other ways. Unfortunately, that's
not possible—but scientists have discovered how to do the next
best thing with quantum dots, which are sometimes known as
"artificial atoms." Simply speaking, they're examples of nanotechnology: groups of atoms
made from semiconductor materials that promise to revolutionize everything from home lights and computer displays to
solar cells and biological warfare detectors. What are they and how do they work? Let's take a closer look!

Photo: Unlike technologies such as LEDs and LCDs, quantum dots can easily make light of
any color.

What are quantum dots?

A quantum dot gets its name because it's a tiny
speck of matter so small that it's effectively
concentrated into a single point (in other words, it's zero-dimensional). As a result, the particles
inside it that carry electricity (electrons and holes, which are places that are missing electrons) are trapped ("constrained") and have well-defined energy levels according to the laws of quantum theory
(think rungs on a ladder), a bit like individual atoms.
Tiny really does mean tiny:
quantum dots are crystals a few nanometers wide, so they're typically
a few dozen atoms across and contain anything from perhaps a hundred to a few
thousand atoms. They're made from a semiconductor such as silicon (a
material that's neither really a conductor nor an insulator, but can
be chemically treated so it behaves like either). And although they're crystals, they behave more like individual
atoms—hence the nickname artificial atoms.

How do quantum dots work?

Quantum dots can be precisely controlled to do all kinds of useful things. School-level physics tells us that if you give an
atom energy, you can "excite" it: you can boost an electron
inside it to a higher energy level. When the electron returns to a
lower level, the atom emits a photon of light with the same energy
that the atom originally absorbed. The color (wavelength and
frequency) of light an atom emits depends on what the atom is; iron
looks green when you excite its atoms by holding them in a hot flame,
while sodium looks yellow, and that's because of the way their energy
levels are arranged. The rule is that different atoms give out
different colors of light. All this is possible because the energy levels
in atoms have set values; in other words, they are quantized.

Artwork: How atoms make light. After absorbing energy (1), an electron inside an atom is promoted to a higher energy level further from the nucleus (2). When it returns, the energy is given out as a photon of light (3). The color of the light depends on the energy levels and varies from one atom to another. Quantum dots produce light in a similar way because the electrons and holes constrained inside them give them similarly discrete, quantized energy levels. However, the energy levels are governed by the size of the dot rather than the substance from which it's made.

Quantum dots do the same trick—they also have quantized energy levels—but dots made from
the same material (say, silicon) will give out different
colors of light depending on how big they are. The biggest quantum
dots produce the longest wavelengths (and lowest frequencies),
while the smallest dots make shorter wavelengths (and higher
frequencies); in practice, that means big dots make red light and
small dots make blue, with intermediate-sized dots producing green light (and the
familiar spectrum of other colors too). The explanation for this is
(fairly) simple. A small dot has a bigger
band gap
(crudely speaking, that's the minimum energy it takes to free electrons so they'll carry electricity through a material), so it takes more energy to excite it; because the frequency of emitted light is
proportional to the energy, smaller dots with higher energy produce
higher frequencies (and shorter wavelengths). Larger dots have more (and
more closely) spaced energy levels, so they give out lower frequencies
(and longer wavelengths).

How do you make a quantum dot?

Quantum dots are precise crystals, so you make
them in much the same way you'd make any other precise semiconductors
crystals. Typical methods include molecular beam epitaxy (MBE, in
which beams of atoms are fired at a "base" or substrate so a single crystal slowly builds up),
ion implantation (where ions are accelerated electrically and fired at a substrate), and
X-ray lithography (a kind of atomic-scale
engraving process using X rays).
Some recent research has been looking into making quantum dots using biological processes, for example, by feeding metals to enzymes.

What can you use quantum dots for?

Optical applications

So far, quantum dots have attracted most interest
because of their interesting optical properties: they're being used
for all sorts of applications where precise control of colored light
is important. In one simple and relatively trivial application, a
thin filter made of quantum dots has been developed so it can be
fitted on top of a fluorescent or LED lamp
and convert its light from a blueish color to a warmer, redder, more attractive shade
similar to the light produced by old-fashioned incandescent lamps.
Quantum dots can also be used instead of pigments and dyes. Embedded
in other materials, they absorb incoming light of one color and give
out light of an entirely different color; they're brighter and more
controllable than organic dyes (artificial dyes made from synthetic chemicals).

Still in the world of optics, quantum dots are being hailed
as a breakthrough technology in the development of more efficient
solar cells. In a traditional solar cell, photons of sunlight knock
electrons out of a semiconductor into a circuit, making useful
electric power, but the efficiency of the process is quite low.
Quantum dots produce more electrons (or holes) for each photon that
strikes them, potentially offering a boost in efficiency of perhaps
10 percent over conventional semiconductors. CCDs (charge-coupled
devices) and CMOS sensors, which are the image-detecting chips in such things as
digital cameras and
webcams, work in a similar way to solar cells, by
converting incoming light into patterns of electrical signals; efficient
quantum dots could be used to make smaller and more efficient image sensors
for applications where conventional devices are too big and clumsy.

Quantum dots are also finding their way into
computer screens and displays, where they offer three important
advantages. First, in a typical LCD (liquid crystal display screen),
the image you see is made by tiny combinations of red, blue, and
green crystals (effectively color filters that switch on and off
under electronic control) that are illuminated from behind by a very
bright backlight. Quantum dots can be tuned to give off light of any
color, so the colors of a quantum dot display are likely to be much
more realistic. Second, quantum dots produce light themselves so they
need no backlight, making them much more energy efficient (an
important consideration in portable devices such as cellphones where
battery life is very important). Third, quantum dots are much smaller
than liquid crystals so they'd give a much higher-resolution image.
Quantum dots are also brighter than a rival technology known as
organic LEDs (OLEDs) and could
potentially make OLED displays obsolete.

Artwork: Quantum dot TV: quantum dots can be used to make the red, green, and blue pixels in TV screens with brighter and more precise colors than in traditional LCDs or rival technologies such as OLEDs.

Quantum computing

Computers get faster and smaller every year, but a
time will come when the physical limits of materials prevent them
advancing any further, unless we develop entirely different technologies.
One possibility would be to store and transmit information with light instead of electrons—a technology broadly
known as photonics. Optical computers could use quantum
dots in much the same way that electronic computers use
transistors (electronic switching devices)—as the basic components in
memory chips and
logic gates.

Optical computers may or may not take off, depending on how much progress computer scientists make with
rival technologies, including quantum computers. In a quantum
computer, bits (binary digits) are stored not by transistors
but by individual atoms, ions, electrons, or photons linked together ("entangled") and acting as
quantum bits called qubits. These quantum-scale "switches"
can store multiple values simultaneously and work on different
problems in parallel. Individual atoms and so on are hard to control
in this way, but quantum dots (on a considerably larger scale) would be much easier to
work with.

Biological and chemical applications

Quantum dots are also finding important medical
applications, including potential cancer treatments.
Dots can be designed so they accumulate in particular parts of the
body and then deliver anti-cancer drugs bound to them. Their
big advantage is that they can be targeted at single organs,
such as the liver, much more precisely than conventional drugs, so reducing the
unpleasant side effects that are characteristic of untargeted, traditional chemotherapy.

Quantum dots are also being used in place of organic dyes in biological research; for example, they can be used like nanoscopic light bulbs to light up and color specific cells that need to be studied under a microscope. They're also being tested as sensors for chemical and biological warfare agents such as anthrax. Unlike organic dyes, which operate over a limited range of colors and degrade relatively quickly, quantum dyes are very bright, can be made to produce any color of visible
light, and theoretically last indefinitely (they are said to be photostable).

Who invented quantum dots?

Quantum dots were discovered in solids (glass crystals) in 1980 by Russian physicist Alexei Ekimov while working at the Vavilov State Optical Institute. In late 1982, American chemist Louis E. Brus, then working at Bell Laboratories (and now a professor at Columbia University), discovered the same phenomenon in colloidal solutions (where small particles of one substance are dispersed throughout another; milk is a familiar example). He discovered that the wavelength of light emitted or absorbed by a quantum dot changed over a period of days as the crystal grew, and concluded that the confinement of electrons was giving the particle quantum properties. These two scientists shared the Optical Society of America's 2006 R.W. Wood Prize for their pioneering work.

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Books

Introductions to quantum theory

Mr Tompkins in Paperback by George Gamov. Cambridge University Press, 1993. What would happen if everyday life worked according to quantum theory's predictions? A good primer for adults and science-minded students aged 16+.

Six Easy Pieces by Richard Feynman. Basic Books, 2011. There's a short introduction to the quantum world in chapter 6.